On a cold, dark Thursday night at 10:00 p.m., a blue and white Con Edison van is parked across the street from the substation on East 13th street, off the Franklin Delano Roosevelt Drive in Manhattan. Two workers in sky blue hard hats get out. They walk over to a manhole and remove the cover. Without a word, one of them grabs a hose from the van and lowers it down. The rest of the hose flops over the ground, extending to the sewer drain at the corner of the street. Steaming hot water slowly streams out. Then the men walk back to their van, sit in the front seat and wait as they calmly deal with the largest storm-related outage in Con Edison’s history.

Con Edison’s East 13th street substation in Lower Manhattan was built to endure a 12.5-foot storm surge. But when Hurricane Sandy hit the Big Apple Monday night, a 14-foot wall of seawater inundated the area, causing a short circuit and an explosion. As a result, more than 220,000 customers were plunged into darkness in Lower and Mid-Manhattan— the area below 39th street on the East Side and below 31st street on the West Side. Across the system — that is, all five boroughs of the city plus Westchester County — 900,000 customers lost power. For comparison, 230,821 were plunged into darkness as a result of a 9.5-foot storm surge that flooded Battery Park during Hurricane Irene in August 2011.

Con Edison’s underground power system is resilient enough to withstand heavy rain, but not the kind of deluge from Sandy, according to spokesperson Allan Drury. For the first time, more than 1,600 external contractors and mutual aid workers from as far west as California have come to the city to help with Con Edison’s restoration effort. Drury said enormous amounts of seawater had to be pumped out, and all of the equipment that came in contact with salt needs to be either dried out, repaired or replaced.

Electricity and water — especially seawater — do not mix. “If you put two wires in normal drinking water, they may not short circuit as easily as when you have salt in the water because salt functions as a conductor,” said Dr. Saifur Rahman, the Joseph Loring Professor of Electrical & Computer Engineering at Virginia Tech. “Salt water damages electrical equipment very easily.” Dr. Roger Anderson, a consultant for Con Edison and adjunct professor at Columbia University’s Lamont-Doherty Earth Observatory, told NPR that the underground cables can withstand some salt. After all, workers salt the streets in the winter time so that cars and pedestrians do not slip. But it will take a while to clean out the large amount of salt that the flood waters brought in from the harbor.

Rahman said that if Con Edison did not preemptively shut down power in Lower Manhattan, the company would have had to replace more cables instead of just repairing them and drying them out. Con Edison actually started preemptively shutting off power in Lower Manhattan Monday evening and, according to Brian Wolff, Senior Vice President at the Edison Electrical Institute, the trade association of shareholder-owned electrical companies, if Con Edison had not done that, repair efforts could have easily taken another week or more. “Some parts need to be disassembled and cleaned manually, power cables need to be replaced, fans and blowers are drying them out,” he said. “It takes much longer to restore an underground substation that transfers power from the grid to customers than a downed power line that has been knocked down by wind.” Con Edison’s goal to get power back to all of Lower and Mid-Manhattan by the end of the day Saturday would be “record-breaking time,” said Wolff.

Con Edison hopes to restore power to the southeastern tip of Manhattan by later Friday, and the rest of Lower and Mid-Manhattan by Saturday night. [Updated: At 5:30 p.m., Con Edison announced that power was restored to 67,000 customers in the East Village and the Lower East Side.] The utility restored power to 2,000 Lower Manhattan customers Wednesday afternoon. The New York Stock Exchange building re-opened earlier that day, and on Thursday night, construction generators at the World Trade Center site were turned back on. The country has come some way toward improving its response, Wolff argued. “When Hurricane Andrew hit in 1992, we did not even have the kind of nationwide mutual aid program that we have today,” he explained, referring to the contractors and utility workers from other states who have flocked to the New York area to speed up Con Edison’s recovery process. However, the electrical equipment in some buildings, especially the ones closest to the Hudson River, are so damaged that they will no longer be able to take in power, according to Con Edison spokesman Allan Drury. Con Edison has isolated those customers so they will not receive electricity when the company turns on the system again.

Still, not having power for five days can seem like an unusually long time for Manhattan residents. “This storm attacked us by sea as well as by air, and folks think because the system is underground, they’re less vulnerable,” Drury said. The flood waters had other impurities from the streets and the rivers, and that is why it has taken so long to clean or replace damaged cables, said Dr. Gregory Reed, former Con Edison employee and electrical engineering professor at the University of Pittsburgh. “All utilities companies have spares for crises, but only so many,” Reed told TIME.

Dr. Rahman suggested that buildings in flood-prone Lower Manhattan could start putting generators higher up so that flood waters cannot reach them. “Many of these buildings in Lower Manhattan have their transformers and backup generator in the basement, in a secure underground space like a bank vault,” he said. “We put things in the basement for safety, but we never thought of the basement getting flooded.” He pointed out that the Goldman Sachs building never lost power because its generator is on top of the building — and suggests more buildings should do that in the future. “Lower Manhattan, because of September 11th, has gone through a lot of safety and security upgrades but did not take into account flooding situations.”

At the same time, Lower and Mid-Manhattan residents might consider themselves lucky — or luckier. Across the Hudson River, just over a million Jersey Central Power & Light customers were without power at the storm’s peak. Some lost power when the storm first hit, got it restored, and then lost it again. JCP&L had a staff of 6,400 in the field, including linemen and forestry workers, working 16 hour shifts. JCP&L spokesperson Christopher Eck said the company hopes to restore power to a majority of its residents by Nov. 7, and restore even more by Nov. 14. However, even that timetable may be too optimistic, considering the devastation in Ocean County and Monmouth County, the hardest hit areas. As JCP&L Spokesperson Eck put it simply: “Islands have been erased, so trying to say that we’ll restore them is a fool’s errand because the entire civilization there has to be rebuilt first. There are no roads and bridges, so we can’t string wire out there in two weeks.”